1. Field of the Invention
This invention relates generally to a method for purifying a crude naphthalene dicarboxylic acid and more particularly concerns a method for purifying a crude naphthalene dicarboxylic acid produced by the liquid-phase oxidation of a dialkylnaphthalene or partially oxidized derivative thereof in a solvent.
2. Discussion of the Prior Art
Fibers and films produced from polyethylenenaphthalate have improved strength and thermal properties relative to fibers and films produced from polyethyleneterephthalate and are especially useful in applications such as tire cords, magnetic tape backings and hot-fill containers. Naphthalene dicarboxylic acid, especially 2,6-naphthalene dicarboxylic acid, is employed as a monomer in the production of polyethylene naphthalate and is typically prepared by the catalyzed, liquid-phase oxidation of a dialkylnaphthalene, especially 2,6-dialkylnaphthalene.
The presence of impurities in the naphthalene dicarboxylic acid can obviously have a serious adverse effect on the physical or chemical properties or performance characteristics of any formulation containing the naphthalene dicarboxylic acid itself or any polymer formed from the naphthalene dicarboxylic acid. In addition, impurities in the naphthalene dicarboxylic acid can adversely affect polymerization processes to which the naphthalene dicarboxylic acid is subjected. Such impurities in the naphthalene dicarboxylic acid formed by the catalyzed, liquid-phase oxidation of a dialkylnaphthalene or partially oxidized derivative thereof are often organic impurities or by-products formed during the oxidation and inorganic impurities corresponding to metal components of the catalysts employed in the oxidation or formed therefrom.
Thus, removal of such impurities from the naphthalene dicarboxylic acid is highly desirable. However, the removal of organic and inorganic impurities from aromatic polycarboxylic acids formed by the catalyzed, liquid-phase oxidation of polyalkyl aromatics is typically very difficult, and the removal technique employed depends on the specific aromatic polycarboxylic acid from which the impurities are to be removed and the specific oxidation conditions and catalyst employed to make it.
For example, Gallivan et al., U.S. Pat. No. 3,592,847 disclose a process for the purification of terephthalic acid by dissolving the crude terephthalic acid in a solvent comprising at least 4.4 molar proportions of an anhydride of a lower alkanoic acid and by adding more than two molar proportions of a lower alkanoic acid per molar proportion of crude terephthalic acid. The lower alkanoic acid can be present in the solvent during dissolution of the terephthalic acid. Alternatively, sufficient water can be added to an anhydride solution containing excess anhydride to form the required amount of the alkanoic acid by reaction with the excess anhydride. Advantageously, the lower alkanoic acid produced as the terephthalic acid dissolves in the anhydride is removed during the dissolution, for example, by treating the mixture with a ketene to convert the acid formed back to the anhydride or by distillation of acid formed, in order to increase the amount of terephthalic acid dissolved. If desired, the terephthalic acid solution can be treated with a decolorizing agent such as adsorbent carbon prior to precipitating the purified product. However, a similar technique has not been disclosed or suggested for the purification of naphthalene dicarboxylic acids, which are much more difficult to purify.
Formation of the methyl ester is the best method known for purifying a naphthalene dicarboxylic acid. However, formation of the methyl ester alone does not afford a sufficiently pure monomer, and the methyl ester itself must be further purified, which purification can be time-consuming and involve relatively complex reaction schemes. Therefore, alternative methods of purification are highly desirable. In particular, removal of the impurities directly from the naphthalene dicarboxylic acids themselves offers the potential advantages of efficiency and economy. However, naphthalene dicarboxylic acids are especially difficult to purify because of their low solubility in most solvents.